Heat-resistant polymer foam, process for producing the same, and foam substrate

ABSTRACT

A heat-resistant polymer foam is disclosed which has excellent heat resistance, a fine cellular structure, and a low apparent density. The heat-resistant polymer foam comprises a heat-resistant polymer having a glass transition point of 120° C. or higher, e.g., a polyimide or polyether imide, and has an average cell diameter of from 0.01 μm to less than 10 μm. This heat-resistant polymer foam can be produced by, for example, impregnating a heat-resistant polymer under pressure with an non-reactive gas such as carbon dioxide, which is in, e.g., a supercritical state, reducing the pressure, and then heating the polymer at a temperature exceeding 120° C. to foam the polymer.

FIELD OF THE INVENTION

[0001] The present invention relates to a polymer foam having fine cellsand excellent heat resistance, a process for producing the same and afoam substrate. The polymer foam is exceedingly useful as, for example,an internal electrical insulator, cushioning material or heat insulatorfor electronic appliances and others, and the foam substrate is usefulas a circuit substrate.

BACKGROUND OF THE INVENTION

[0002] Conventional general processes for foam production includechemical processes and physical processes. In the chemical processes, acompound (blowing agent) added to a polymer base is thermally decomposedand cells are formed by the resultant gas to obtain a foam. However,this foaming technique has a disadvantage that after the gas generation,a residue of the blowing agent tends to remain in the foam. Thistechnique therefore poses a problem concerning fouling by corrosivegases or impurities especially when the foam is used as an electronicpart or the like because fouling prevention is highly required in suchapplications.

[0003] On the other hand, a general physical process comprisesdispersing a low boiling liquid (blowing agent) such as achlorofluorocarbon or hydrocarbon into a polymer and then heating thepolymer to volatilize the blowing agent, thereby forming cells. Forexample, U.S. Pat. No. 4,532,263 discloses a method for obtaining afoamed polyether imide or another foamed polymer using methylenechloride, chloroform, trichloroethane or the like as a blowing agent.However, this foaming technique has problems concerning the harmfulnessof the substances used as a blowing agent and various influences thereofon the environment, including ozonosphere depletion. In addition, it isdifficult to obtain with this technique a foam having fine cells uniformin diameter, although the technique is generally suitable for obtaininga foam having a cell diameter of several tens of micrometers or larger.

[0004] Recently, a technique for obtaining a foam having a small celldiameter and a high cell density was proposed which comprises dissolvinga gas such as nitrogen or carbon dioxide in a polymer at a highpressure, subsequently releasing the polymer from the pressure, andheating the polymer to a temperature around the glass transitiontemperature or softening point of the polymer to thereby form cells.This foaming technique, in which nuclei are formed in athermodynamically unstable state and are allowed to expand and grow tothereby form cells, has an advantage that a foam having a finelycellular novel structure is obtained. For example, application of thistechnique to a styrene resin having a syndiotactic structure isdisclosed in JP-A-10-45936 (the term “JP-A” as used herein means an“unexamined published Japanese patent application”). Specifically, thisreference discloses a method for obtaining a molded foam having closedcells with a cell size of from 0.1 to 20 μm. There is a descriptiontherein to the effect that this molded foam is useful as an electriccircuit member. However, this molded foam deforms or bends when used attemperatures not lower than 100° C., because styrene resins having asyndiotactic structure generally have a glass transition point around100° C. Consequently, applications of this molded foam are limited to anarrow range.

[0005] JP-A-6-322168 discloses a method which comprises heating apressure vessel containing a thermoplastic polymer, e.g., a polyetherimide, to or around the Vicat softening point of the polymer,impregnating the heated polymer with a gas in a supercritical fluidstate, and then releasing the polymer from the pressure to obtain aporous foamed article having a low density. However, this method has thefollowing drawback. Since the polymer is heated to or around the Vicatsoftening point thereof for impregnation with a gas in a high-pressurevessel, the gas readily expands upon pressure decrease because thepolymer is in a molten state. The resultant foam hence has a cell sizeas large as about from 10 to 300 μm. In the case where this foam islaminated with a metal foil to produce a laminate for use as a circuitsubstrate, pattern formation on the metal foil side by etching islimited in the degree of pattern fineness. In addition, the foam isexpected to further have a problem that chemicals used for theprocessing, such as a resist, an etchant, and a stripping fluid,infiltrate into pores of the foam to considerably reduce electricalreliability.

SUMMARY OF THE INVENTION

[0006] Accordingly, an object of the invention is to provide aheat-resistant polymer foam having excellent heat resistance, a finecellular structure, and a low relative density.

[0007] Another object of the present invention is to provide a processfor producing the heat-resistant polymer foam.

[0008] Still another object of the invention is to provide a foamsubstrate which has a metal foil where a fine pattern can be formed andwhich is useful as a circuit substrate having high electricalreliability.

[0009] As a result of intensive studies to accomplish the above objects,it has been found that a foam having excellent heat resistance andexceedingly fine cells is obtained by impregnating a heat-resistantpolymer with a non-reactive gas such as carbon dioxide under pressure,reducing the pressure, and then heating the polymer at a specifictemperature. The present invention has been completed based on thisfinding.

[0010] The present invention provides a heat-resistant polymer foamwhich comprises a heat-resistant polymer and has an average celldiameter of from 0.01 μm to less than 10 μm. The heat-resistant polymerhas a glass transition point of, e.g., 120° C. or higher. Thisheat-resistant polymer includes a polyimide, a polyether imide and thelike.

[0011] The present invention further provides a process for producing aheat-resistant polymer foam which comprises impregnating aheat-resistant polymer with a non-reactive gas under pressure, reducingthe pressure, and then heating the impregnated polymer at a temperatureexceeding 120° C. to foam the polymer. The heating for foaming ispreferably conducted at a temperature at which the heat-resistantpolymer in an unfoamed state has a modulus of elasticity of 1×10⁷ Pa orhigher. The heat-resistant polymer to be foamed has a glass transitionpoint of, e.g., 120° C. or higher. The heat-resistant polymer may be,e.g., a polymer selected from polyimides and polyether imides. Thenon-reactive gas is, for example, carbon dioxide. The heat-resistantpolymer may be impregnated with the non-reactive gas in a supercriticalstate.

[0012] The present invention also provides a foam substrate whichcomprises a foamed resin layer comprising the heat-resistant polymerfoam and a metal foil layer disposed on at least one side of the resinlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a scanning electron micrograph showing a sectionalstructure of the foamed sheet obtained in Example 1.

[0014]FIG. 2 is a scanning electron micrograph showing a sectionalstructure of the foamed sheet obtained in Example 2.

[0015]FIG. 3 is a scanning electron micrograph showing a sectionalstructure of the foamed sheet obtained in Example 3.

[0016]FIG. 4 is a scanning electron micrograph showing a sectionalstructure of the foamed sheet obtained in Comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The polymer used as the material of the foam of the presentinvention is not particularly limited, and either amorphous andcrystalline polymers can be used so long as it has heat resistance.Examples of the polymer include polypropylene, polyacetals, polyamides,polymethylpentene, polycarbonates, poly (butylene terephthalate), poly(ethylene terephthalate), poly(phenylene sulfide), polysulfones,polyethersulfones, polyetheretherketones, poly(vinylidene fluoride),polytetrafluoroethylene, poly (amide-imide)s, polyimides andpolyetherimides. However, the polymer to be foamed should not beconstrued as being limited to these examples. Especially advantageouspolymers have a glass transition point of 120° C. or higher. Suchpolymers can be used alone or as mixtures of two or more thereof.

[0018] Of the above polymers, polyimides and polyether imides areparticularly preferably used. Polyimides can be obtained by theconventional methods. For example, a polyimide can be obtained byreacting an organic tetracarboxylic dianhydride with a diamino compound(diamine) to prepare a polyimide precursor (poly(amic acid)) andsubjecting this polyimide precursor to dehydrocyclization.

[0019] Examples of the organic tetracarboxylic dianhydride includepyromellitic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride,2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,2,2-bis(3,4-dicarboxyphenyl)-1,1,1,1,3,3,3-hexafluoropropanedianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl) ether dianhydride and bis (3,4-dicarboxyphenyl)sulfone dianhydride. These organic tetracarboxylic dianhydrides can beused alone or as mixtures of two or more thereof.

[0020] Examples of the diamino compound include m-phenylenediamine,p-phenylenediamine, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenylether, 4,4′-diaminodiphenyl sulfone, 3,3′-diaminodiphenyl sulfone,2,2-bis(4-aminophenoxyphenyl)propane,2,2-bis(4-aminophenoxyphenyl)hexafluoropropane,1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene,2,4-diaminotoluene, 2,6-diaminotoluene, diaminodiphenylmethane,4,4′-diamino-2,2-dimethylbiphenyl and2,2-bis(trifluoromethyl)-4,4′-diaminobiphenyl.

[0021] The polyimide precursor is obtained by reacting an organictetracarboxylic dianhydride with a diamino compound (diamine) in anapproximately equimolar proportion usually in an organic solvent at 0 to90° C. for about from 1 to 24 hours. Examples of the organic solventinclude polar solvents such as N-methyl-2-pyrrolidone,N,N-dimethylacetamide, N,N-dimethylformamide and dimethyl sulfoxide.

[0022] The dehydrocyclization reaction of the polyimide precursor isconducted, for example, under heating or by the action of adehydrocyclizing agent such as a mixture of acetic anhydride withpyridine. In general, polyimides are insoluble in organic solvents andhave poor moldability. Consequently, in many cases the polyimideprecursor is formed into a film or sheet or another form before beingsubjected to dehydrocyclization to obtain a polyimide molded article.

[0023] Besides the above method, a polyimide can be obtained by othermethods including: a method of reacting an organic tetracarboxylicdianhydride with an N-silylated diamine to obtain silyl ester of apoly(amic acid) and cyclizing the resulting silyl ester of a poly(amicacid) under heating; a method of reacting an organic tetracarboxylicdianhydride with a diisocyanate; a method of reacting an organictetracarboxylic dithiodianhydride with a diamino compound; and a methodof subjecting an organic tetracarboxydiimide and a diamino compound toimide exchange reaction.

[0024] The polyether imides can also be obtained by the conventionalmethod. However, commercially available polyether imide products may beused, such as Ultem (manufactured by General Electric Co.) and Superio(manufactured by Mitsubishi Plastics Industries Ltd.).

[0025] The heat-resistant polymer to be foamed in the process of thepresent invention may contain additives according to need. Thoseadditives are not particularly limited in kind, and various additivesfor general use in expansion molding can be used. Examples the additivesinclude nucleating agents for cell formation, nucleating agents forcrystal formation, plasticizers, lubricants, colorants, ultravioletabsorbers, antioxidants, fillers, reinforcements, flame retardants andantistatic agents. Although the amount of such additives added is notparticularly limited, they are desirably added in an amount used in theconventional molding of thermoplastic resins.

[0026] The gas used as a blowing agent in the process of the presentinvention is not particularly limited so long as it is non-reactive withthe heat-resistant polymer and the polymer can be impregnated with thegas. Examples the gas include carbon dioxide, nitrogen gas and air.Those gases may be used alone or as mixtures of two or more thereof. Ofthose, carbon dioxide is particularly preferably used for the reasonthat the heat-resistant polymer used as a material of foam can beimpregnated with carbon dioxide in a larger amount and at a higher ratethan other gases.

[0027] From the standpoint of increasing the impregnation rate of thepolymer with the gas, the gas is preferably used in a supercriticalstate. For example, in the case of carbon dioxide, the solubility ofcarbon dioxide in the polymer can be greatly enhanced and an increasedcarbon dioxide concentration in the polymer is attainable, when thecarbon dioxide, whose critical temperature and critical pressure are 31°C. and 7.4 MPa, respectively, is in a supercritical state having atemperature of 31° C. or higher and a pressure of 7.4 MPa or higher.Furthermore, the infiltration of a gas in a supercritical state has thefollowing advantage. Since the gas concentration in the polymer is high,an abrupt pressure drop results in the generation of a large amount ofcell nuclei. These cell nuclei grow to yield cells, and the cell densityin this foam is higher than those in foams having the same porosity asthat. Namely, exceedingly fine cells can be obtained.

[0028] The process of the present invention comprises: a gasimpregnation step in which the heat-resistant polymer is impregnatedwith a non-reactive gas under pressure; a pressure reduction step (stepof releasing the polymer from pressure) in which the pressure is reducedafter the impregnation step; and a heating/foaming step in which theimpregnated polymer is foamed by heating. Those steps may be conductedbatchwise or continuously.

[0029] In a batch process, a foam can be produced, for example, in thefollowing manner. A resin composition containing a heat-resistantpolymer is extruded with an extruder, e.g., a single- or twin-screwextruder, to form a sheet containing the heat-resistant polymer as abase resin. Alternatively, a resin composition containing aheat-resistant polymer is uniformly kneaded with a kneading machinehaving blades of the roller, cam, kneader, or Banbury type, and thekneaded composition is press-molded in a given thickness by hot pressingor another means to form a sheet containing the heat-resistant polymeras a base resin. The unfoamed sheet thus obtained is placed in ahigh-pressure vessel. A non-reactive gas comprising, e.g., carbondioxide, nitrogen or air is forced into the vessel to impregnate theunfoamed sheet with the non-reactive gas. At the time when the sheet hasbeen sufficiently impregnated with the non-reactive gas, the pressure isreduced (usually to atmospheric pressure) to generate cell nuclei in thebase resin. These cell nuclei are heated to grow cells. The sheet isthen rapidly cooled with,e.g., cold water to terminate the cell growthand fix the shape. Thus, a heat-resistant polymer foam is obtained.

[0030] On the other hand, a continuous process can be conducted, forexample, in the following manner. While a resin composition containing aheat-resistant polymer is kneaded with an extruder, e.g., a single- ortwin-screw extruder, a non-reactive gas is forced into the extruder tosufficiently impregnate the resin with the non-reactive gas. The resinis then extruded to thereby reduce the pressure (usually to atmosphericpressure) and generate cell nuclei. The extrudate is heated to therebygrow cells and is then rapidly cooled with, e.g., cold water toterminate the cell growth and fix the shape. Thus, a heat-resistantpolymer foam can be obtained.

[0031] In the gas impregnation step, the pressure can appropriately beselected, taking the kind of the gas, operation property, etc., intoconsideration. However, in the case of using carbon dioxide, forexample, the pressure is about from 5 to 100 MPa (preferably about from7.4 to 100 MPa). The temperature in the gas impregnation step variesdepending on the kind of the gas used, the glass transition temperatureof the polymer, etc., and can be selected in a wide range. However, toohigh temperature during impregnation tends to result in large celldiameter. For this reason, the temperature is preferably in the range offrom 10° C. to lower than 120° C., or from 10° C. to lower than theglass transition temperature of the heat-resistant polymer.

[0032] An important feature of the process according to the presentinvention resides in that the temperature at which the polymer is heatedin the heating/foaming step is higher than 120° C. If the temperaturefor heating is 120° C. or lower, it is difficult to obtain a foam havingfine cells and a low relative density. The temperature for heating ispreferably about 150° C. or higher, more preferably about 170° C. orhigher. It is desirable that the temperature for heating be higher thanthe glass transition point of the heat-resistant polymer to be foamed.

[0033] In a preferred embodiment of the process according to the presentinvention, the temperature for heating is regulated to a temperature atwhich the heat-resistant polymer to be foamed has, in an unfoamed state,a modulus of elasticity (varying with temperature) of 1×10⁷ Pa or higher(e.g., about from ×10⁷ to ×10¹¹ Pa). Heating at a temperature in a rangewhere the modulus of elasticity is lower than 1×10⁷ Pa is undesirable inthat the polymer becomes too soft and this results in cases where cellsgrow excessively to cause gas leakage, making it impossible to obtain afoam, and in cases where cell coalescence occurs, resulting in anextraordinarily reduced cell density. Also, there are cases where thepolymer deforms during cell growth.

[0034] The shape of the heat-resistant polymer to be foamed is notlimited to sheet or film, and the polymer may be a molded product ofanother shape, e.g., a columnar or spherical shape. Although a foamcomprising a polyimide can be obtained by foaming a molded product,e.g., film, of a polyimide, it can also be obtained by thermallyconverting the polyimide precursor in a film or another molded form intoa polyimide after the precursor has been foamed or during the foaming.

[0035] The heat-resistant polymer foam thus obtained not only hasexcellent heat resistance but has uniform and fine cells and a lowrelative density. For example, the average cell diameter of the foam isfrom 0.01 μm to less than 10 μm, preferably about from 0.02 μm to 5 μm.The density of the heat-resistant polymer foam is, for example, from 0.4to 1.4 g/cm³, preferably about from 0.5 to 1.25 g/cm³, although itvaries depending on the kind of the heat-resistant polymer.

[0036] As described above, the heat-resistant polymer foam of thepresent invention has high heat resistance, uniform and fine cells and alow dielectric constant. Consequently, the foam can be utilized as, forexample, an internal electrical insulator, cushioning material, heatinsulator or circuit substrate for electronic appliances and otherdevices while taking advantage of excellent properties inherent in theheat-resistant polymer, such as heat resistance, mechanical properties,wearing resistance and high resilience. The foam is especially suitablefor use as a thin sheet.

[0037] The foam substrate of the present invention can be produced byforming a metal foil layer on one or each side of a foamed resin layercomprising the heat-resistant polymer foam. Although the metal foil isnot particularly limited, foils generally used include a stainless steelfoil, copper foil, aluminum foil, copper-beryllium foil,phosphorus-bronze foil and iron-nickel alloy foil. Methods for forming ametal foil layer are not particularly limited, and examples thereofinclude (1) a method in which a resin layer to be foamed is formed on asubstrate comprising a metal foil and then foamed, and (2) a method inwhich a foamed resin layer is produced first and is then metallized by aconventional technique, e.g., sputtering, electroplating or electrolessplating. Two or more techniques may be used in combination. The foamsubstrate thus obtained is useful as a circuit substrate.

[0038] The heat-resistant polymer foam of the present invention hasexcellent heat resistance, a low relative density and an exceedinglysmall cell size. Furthermore, the foam can have a reduced dielectricconstant because the resin has cells therein.

[0039] According to the process of the present invention, aheat-resistant polymer foam having such excellent properties can easilybe obtained efficiently.

[0040] According to the foam substrate of the present invention, sincethe cell size of the foam is exceedingly small, not only a fine patterncan be formed in the metal foil, but also chemicals such as a resist, anetchant and a stripping fluid are difficult to infiltrate into the finecells during the processing for pattern formation. Consequently, highelectrical reliability is obtained.

[0041] The present invention will be explained in detail below byreference to the following Examples, but the invention should not beconstrued as being limited to those Examples in any way.

[0042] The modulus of elasticity, sectional structure and dielectricconstant of each sheet were determined or examined by the followingmethods.

Determination of Modulus of Elasticity

[0043] Using DMS-210, manufactured by Seiko Instruments Inc., each sheetwas heated at a temperature rising rate of 5° C./min with vibration at10 Hz in the air to obtain a curve of storage elastic modulus (E′) forthe sheet. The value of that modulus was taken as modulus of elasticity.

Examination of Sheet Structure

[0044] Each foamed sheet produced was frozen with liquid nitrogen andthen broken. A section resulting from the breakage was examined with ascanning electron microscope (SEM; Hitachi S-570) at an acceleratingvoltage of 10 kV.

Determination of Dielectric Constant

[0045] The dielectric constant of each sheet was determined with HP4248A Precision LCR Meter, manufactured by Yokokawa Hewlett-Packard Co.,Ltd.

Synthesis Example

[0046] Synthesis of Polyimide Precursor [BPDA/PDA]

[0047] 27 g of p-phenylene diamine (PDA) was introduced into a 500 mlseparable flask equipped with a stirrer and a thermometer. 392 g ofN-methyl-2-pyrrolidone (NMP) was added to the flask. The contents in theflask were stirred to dissolve the PDA. 73.5 g of3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) was graduallyintroduced into the flask. The contents in the flask were thencontinuously stirred at a temperature of 30° C. or lower for 2 hours toobtain a polyimide resin precursor solution having a concentration of20% by weight. This polyimide resin precursor solution had an intrinsicviscosity (as measured at 30° C. in NMP at a concentration of 0.5 g/100ml) of 1.5 and a solution viscosity at 30° C. of 800 Pa·s.

EXAMPLE 1

[0048] A 188 μm thick polyether imide film (Superio UT Type F;manufactured by Mitsubishi Plastics Industries Ltd.; T_(g), 226° C.) wascut into a circular sheet having a diameter of 80 mm. This sheet wasplaced in a 500 cc pressure vessel and maintained for 1 hour in a carbondioxide atmosphere having a temperature of 40° C. and a pressure of 25MPa to thereby impregnate the sheet with carbon dioxide. After thepressure was reduced to atmospheric pressure, the sheet was immediatelyimmersed in a 200° C. oil bath for 30 seconds to grow cells, rapidlytaken out of the bath and then quenched with ice water. Thus, aheat-resistant foam made of the polyether imide was obtained. This foamhad a density of 0.684 g/cm³ and an average cell diameter as determinedfrom an SEM image of 0.13 μm. The sheet, before being foamed, had amodulus of elasticity at 200° C. of 1.8×10⁹ Pa.

EXAMPLE 2

[0049] A 188 μm thick polyether imide film (Superio UT Type F;manufactured by Mitsubishi Plastics Industries Ltd.; T_(g), 226° C.) wascut into a circular sheet having a diameter of 80 mm. This sheet wasplaced in a 500 cc pressure vessel and maintained for 30 minutes in acarbon dioxide atmosphere having a temperature of 40° C. and a pressureof 25 MPa to thereby impregnate the sheet with carbon dioxide. After thepressure was reduced to atmospheric pressure, the sheet was immediatelyimmersed in a 200° C. oil bath for 30 seconds to grow cells, rapidlytaken out of the bath and then quenched with ice water. Thus, aheat-resistant foam made of the polyether imide was obtained. This foamhad a density of 0.769 g/cm³ and an average cell diameter as determinedfrom an SEM image of 1.2 μm.

EXAMPLE 3

[0050] The polyimide resin precursor solution obtained in SynthesisExample was applied with an applicator to an Alloy 42 foil having athickness of 20 μm. The coating was pre-dried in a circulating hot-airdrying oven at 150° C. for 1 hour. This film layer was heated first at200° C. for 30 minutes and then at 250° C. for 30 minutes to therebyproduce a sheet composed of the aluminum foil and a polyimide film layerformed thereon. This polyimide had a glass transition point (Tg) of 180°C. or higher.

[0051] The sheet thus produced was cut into a 50×50 mm size. This cutsheet was placed in a 500 cc pressure vessel and maintained for 90minutes in a carbon dioxide atmosphere having a temperature of 40° C.and a pressure of 25 MPa to thereby impregnate the sheet with carbondioxide. After the pressure was reduced to atmospheric pressure, thesheet was immediately immersed in a 300° C. oil bath for 30 seconds togrow cells, rapidly taken out of the bath and then quenched with icewater. Thus, a sheet composed of the aluminum foil and, formed thereon,a heat-resistant foam made of the polyimide was obtained. This foam hada density of 1.16 g/m³ and an average cell diameter as determined froman SEM image of 0.057 μm. The sheet, before being foamed, had a modulusof elasticity at 300° C. of 4.0×10⁹ Pa.

EXAMPLE 4

[0052] A polyether imide (Ultem 1000, manufactured by General ElectricCo.) was dissolved in methylene chloride in an amount so as to be aconcentration of 20% by weight. This solution was spread on a 35 μmthick rolled copper foil in a thickness of 30 μm. The coating waspre-dried at 80° C. for 30 minutes and then heated in a nitrogenatmosphere at 200° C. for 30 minutes to obtain a polyether imide/coppersubstrate. This substrate was cut into a circular sheet having adiameter of 80 mm. This sheet was placed in a 500 cc pressure vessel andmaintained for 30 minutes in a carbon dioxide atmosphere having atemperature of 40° C. and a pressure of 25 MPa to thereby impregnate thesheet with carbon dioxide. After the pressure was reduced to atmosphericpressure, the sheet was immediately immersed in a 200° C. oil bath for30 seconds to grow cells, rapidly taken out of the bath and thenquenched with ice water. Thus, a heat-resistant foam substrate composedof the polyether imide and the copper foil was obtained. The copper foilof the foam substrate was removed by etching, and the density of theresin was measured. As a result, the resin density was found to be 0.77g/cm³. The foamed resin had an average cell diameter as determined froman SEM image of 1.3 μm and a dielectric constant ∈ of 2.2 (1 MHz).

Comparative Example 1

[0053] A 188 μm thick polyether imide film (Superio UT Type F;manufactured by Mitsubishi Plastics Industries Ltd.; T_(g), 226° C.) wascut into a circular sheet having a diameter of 80 mm. This sheet wasimmersed in dichloromethane at room temperature. After 1 hour, the sheetwas taken out. This sheet was immediately immersed in a 200° C. oil bathfor 30 seconds to grow cells, rapidly taken out of the bath and thenquenched with ice water. Thus, a heat-resistant foam made of thepolyether imide was obtained. This foam had a density of 0.771 g/cm³.However, the average cell diameter thereof as determined from an SEMimage was as large as 88 μm, and the cells had extremely poor uniformityin structure.

Comparative Example 2

[0054] A polyether imide (Ultem 1000, manufactured by General ElectricCo.) was dissolved in methylene chloride in an amount so as to be aconcentration of 20% by weight. This solution was spread on a 35 μmthick rolled copper foil in a thickness of 30 μm. The coating waspre-dried at 80° C. for 30 minutes and then heated in a nitrogenatmosphere at 200° C. for 30 minutes to obtain a polyether imide/coppersubstrate. This substrate was cut into a circular sheet having adiameter of 80 mm. This sheet was immersed in a 200° C. oil bath for 30seconds, rapidly taken out of the bath and then quenched with ice water.Thus, a substrate composed of the polyether imide and the copper foilwas obtained. The copper foil of this substrate was removed by etching,and the density of the resin was measured. As a result, the resindensity was found to be 1.27 g/cm³. No cells were observed on an SEMimage. The resin layer had a dielectric constant ∈ of 3.4 (1 MHz).

[0055] As is apparent from the above, the heat-resistant polymer foamsobtained in the Examples each had a cellular structure having a cellsize of from 0.01 μm to less than 10 μm.

What is claimed is:
 1. A heat-resistant polymer foam comprising aheat-resistant polymer, said foam having an average cell diameter offrom 0.01 μm to less than 10 μm.
 2. The heat-resistant polymer foam asclaimed in claim 1 , wherein the heat-resistant polymer has a glasstransition point of 120° C. or higher.
 3. The heat-resistant polymerfoam as claimed in claim 1 , wherein the heat-resistant polymer is apolymer selected from the group consisting of polyimides and polyetherimides.
 4. A process for producing a heat-resistant polymer foam whichcomprises impregnating a heat-resistant polymer with a non-reactive gasunder pressure, reducing the pressure, and then heating the impregnatedpolymer at a temperature exceeding 120° C. to foam the polymer.
 5. Theprocess for producing a heat-resistant polymer foam as claimed in claim4 , wherein the heating for foaming is conducted at a temperature atwhich the heat-resistant polymer in an unfoamed state has a modulus ofelasticity of ×10⁷ Pa or higher.
 6. The process for producing aheat-resistant polymer foam as claimed in claim 4 , wherein theheat-resistant polymer has a glass transition point of 120° C. orhigher.
 7. The process for producing a heat-resistant polymer foam asclaimed in claim 4 , wherein the heat-resistant polymer is a polymerselected from the group consisting of polyimides, polyimide precursorsand polyether imides.
 8. The process for producing a heat-resistantpolymer foam as claimed in claim 4 , wherein the non-reactive gas iscarbon dioxide.
 9. The process for producing a heat-resistant polymerfoam as claimed in claim 4 , wherein the non-reactive gas with which theheat-resistant polymer is impregnated is used in a supercritical state.10. A foam substrate comprising: a foamed resin layer comprising aheat-resistant polymer foam comprising a heat-resistant polymer, saidfoam having an average cell diameter of from 0.01 μm to less than 10 μm,and a metal foil layer formed on at least one side of the resin layer.